The best way to determine how many flights you can safely get is by measuring the voltage at the beginning and end of each flight. Do this a couple of times until you get a "feel" for how much capacity you actually use for each flight. I can't tell you how much it will be since it depends on the type of plane and the length of each flight.

You should also construct a chart of battery voltage over time while you discharge the battery with a known load. Note what voltage the plateau occurs at. Also note the voltage where the batteries drop off rapidly. Don't start a flight unless you have enough capacity plus a generous reserve.

If this sounds like a lot of work, just use a field charger after 3 flights to peak them. That's what I usually do. It gives me a chance to BS a little bit.

Tom